Hearing aids are generally placed at least partially in the ear canal of a user and have the function of amplifying ambient sound entering the ear from the ambient space to the user. A hearing aid is generally having a distal end and a proximal end, and when the distal end of the hearing aid is placed in the ear canal of a user, a residual space is formed between the distal end of the hearing aid and the tympanic membrane in the ear canal. A microphone is generally placed in the proximal end of the hearing aid, so that it is pointing towards the ambient space of the user, and a loudspeaker is generally placed in the distal end of the hearing aid to transmit sound signals from the microphone to the residual space in the ear canal and hence to the tympanic membrane.
Hearing aids are typically equipped with a vent adapted to enable sound pressure equalisation between the ambient space and the residual space. The vent may also prevent occlusion experienced by the user of the hearing aid, which occlusion is caused by enclosed sound waves conducted via the skull and head tissue to the residual space. The vent ensures that the enclosed pressure changes may be equalised with the pressure in the ambient space.
The acoustic properties of the vent may establish a positive feedback loop between the loudspeaker and the microphone. Acoustic feedback may limit the possible amplification level in hearing aids. This problem is accentuated by the use of open hearing aids, i.e. hearing aids with a large vent.
EP 1708544 and U.S. Pat. No. 6,134,329 disclose that a so-called Real-Ear-Measurement (REM) can be performed, when fitting a hearing aid to a user, where the acoustic properties of the vent are measured. First a hearing aid is placed in the ear canal of a user and then a probe-microphone is inserted in the residual space defined between the distal end of the hearing aid and the tympanic membrane of the user to estimate the sound pressure level at the tympanic membrane in the residual space during the fitting of the hearing aid to the user. But since the acoustic model parameters stored in the hearing aid can not be changed and the probe itself causes a change in the residual space and the insertion of the probe may cause leakage, the use of REM may lead to incorrect results, since this solution relates to a static correction.
Various prior art documents, e.g. US 2002/0057814 and US 2005/0094827, describe hearing aids where the acoustic feedback is countered by digital processing algorithms. These algorithms estimate the acoustic transfer function from the loudspeaker to the microphone and perform a partial cancelling of the feedback signal based on the microphone signal and on the estimated transfer function. The transfer function is the mathematical representation of the relation between the microphone (i.e. the input of the system) and the loudspeaker (i.e. the output of the system).
It remains a problem to compensate properly for acoustic feedback in hearing aids, because even when using digital processing algorithms it is not possible to fully account for changes in the acoustic environment of the user.